I recently started using custom shader. Shader can do diffuse and specular lighting and normal mapping. But normal mapping is causing really ugly artifacts (some sort of pixeling noise) for textures in greater distance.

I suspect that those artifacts may be specular highlights because when I tried your shader in place of my normal map' shader, the specualar shading was imitating a light shining at a 90 degree angle about the model to where the diffuse shader's light was shining from. So yeah, hopefully it's just spec highlights, so try this:

Everything up until the Vertex Shader is the same. The VS does exactly the same thing, but I added a pre-shader just because I can.
The Pixel Shader is where it's at. I cleaned it up a little, but the big changes are:

Apart from the float4 to float3, I replaced the variable name 'reflect' with 'reflection' so I could use the HLSL function: reflect. This cool function means you don't have to write out the equation, just pass the 'incident ray' and the normal and it'll do the rest. It also makes sure that it's done correctly, unlike here. You used diffuse instead of dot(-DiffuseDirection, normal) and in diffuse, you normalized -DiffuseDirection.

It should go without saying that reflect has been changed to reflection in the line below as well.

This code worked for the model I used it on, so I'm hoping this is the fix you're after.